26 
BULLETIN 1060, TL S. DEPARTMENT OF AGRICULTURE. 
Table 7. — Average diameter outside bark at 15 feet above ground at various 
ages and average annual diameter groioth in each decade of Sitka spruce 
growing on all sites in Oregon and Washington. 
[Based on measurement 
of 557 dominant trees.] 
(Curved.) 
Age. 
Average 
diameter. 
Average 
annual 
diameter 
growth 
in each 
decade. 
Age. 
Average 
diameter. 
Average 
annual 
diameter 
growth 
in each 
decade. 
Years. 
20 
Inches. 
2.0 
5.6 
9.5 
12.8 
15.7 
18.2 
20.5 
22.5 
24.4 
26.3 
28.1 
29.9 
31.5 
33.1 
34.7 
36.2 
37.7 
39.2 
40.6 
42.0 
Inches. 
Years. 
220 
Inches. 
43.4 
44.8 
46.2 
47.5 
48.8 
50.1 
51.4 
52.7 
54.0 
55.3 
56.6 
57.8 
59.0 
60.2 
61.4 
62.5 
63.6 
64.7 
65.8 
Inches. 
0.14 
14 
30 

36 
39 
33 
29 
25 
23 
20 
19 
19 
18 
18 
16 
16 
16 
15 
15 ! 
15 
14 ! 
14 
230 
40 
240 
.14 
13 
50 
250. . 
60 
260 
.13 
.13 
13 
70 
270 
80 
280 . 
90 
290. . 
13 
100 
300 
13 
110 
310 
.13 
120 
320. . . 
13 
130 „ 
330. . 
. 12 
140 
340 
12 
150 
350 
.12 
160 
360 
.12 
170 
370 . 
.11 
180 
380 
.11 
190 
390 
. 11 
200 
400 
.11 
210 
Diameter measurements at breast height are of little value in a 
Sitka spruce growth study, as this species commonly has a pro- 
nounced basal swell and its root base is usually well above the gen- 
eral ground level, owing to its habit of starting on down logs. For 
these reasons, in Forest Service timber survey work in spruce, diam- 
eters are taken at a point.1 foot above the swell; but this is a variable 
height and can not be used in growth studies when the relation be- 
tween age and diameter is desired. In Table 7, therefore, diameters 
•are given for a distance 15 feet above the ground and on most trees 
this point is above the basal swell. It must be borne in mind, how- 
ever, that this uniform height above ground does not mean a uniform 
distance between this point and the root bases of all the trees meas- 
ured. Trees which started on fallen logs 4 or 5 feet in diameter 
naturally have their root bases 4 or 5 feet above the ground, and the 
number of annual rings showing at the 15-foot point in these trees 
is, of course, less than at this point on trees whose root bases rest 
on the ground. It was found, however, that the discrepancy for all 
the trees measured amounted to only two years. This variation is 
rendered of little consequence by the rapid height growth of Sitka 
spruce in its sapling stage, when 3 feet per year is not an unusual 
growth. Another point that must be kept in mind in this connec- 
tion is that it takes an average of 14 years for the seedling to reach 
a height of 15 feet, as shown by Table 5, and therefore a tree must 
be more than 14 years old before it shows diameter at this point of 
measurement. 
